Справочник Пользователя для National Instruments NI VISION PCI-8254R

Use the Indicator Only option only to test the watchdog timer. If software 

becomes unresponsive, it cannot be relied upon to send notification to the host.

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Indicator Only—This option sends the expiration signal back to the 
development machine through software. True indicates an expired 
watchdog timer. False indicates an unexpired watchdog timer. The 
expiration signal that indicates an expired watchdog timer continues to 
assert until the watchdog timer is disarmed. Disarming the watchdog 
timer resets the software indicator.

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TTL Output 0—This option outputs a signal on TTL Output 0. High 
indicates that the watchdog timer has expired. Low indicates that the 
watchdog timer has not expired. If the watchdog timer has expired, the 
expiration signal continues to assert until the watchdog timer is 
disarmed.

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Shutdown—If Shutdown mode is enabled, the outputs go to the 
user-defined shutdown states.

The isolated trigger inputs on the NI 8254R are current sinking and 
optically isolated. The following are considerations you need to make when 
connecting the digital I/O.

You can wire an isolated input to a sourcing output device, as shown in the 
following figure. Refer to Getting Started with the NI PCI-8254R for 
switching thresholds and current requirements.

Do not apply a voltage greater than 30 VDC to the isolated inputs. Doing so could 

damage the NI 8254R.

Figure 2-5 shows an example of connecting an isolated input to a sourcing 
output device.